A Russian military unit has arrived in Syria, according to Russian news reports, a development that a United Nations Security Council source told US media would have serious repercussions.

Russian Media on the other hand said this was a clear sign by the Russians to foreign powers operating inside Syria to be careful what they do.

Russia, one of President Bashar al-Assad's strongest allies despite US and French condemnation of the government's crackdown on the country's "rebels", has repeatedly blocked the United Nations Security Council's attempts to halt the violence, accusing the U.S. and its allies of trying to start another war.

Now the Russian Black Sea fleet's Iman tanker has arrived in the Syrian port of Tartus on the Mediterranean Sea with an anti-terror squad from the Russian Marines aboard according to the Interfax news agency. The Assad government has insisted it is fighting a terrorist insurgency. The Russian news reports did not elaborate on the Russian troops' mission in Syria or if they are expected to leave the port.

The presence of Russian troops in Syria could be a "pretty obvious" show of support to the regime, according to Russian security expert Mark Galeotti.